Christos S. Mantzoros

Lack of sufficient nutrition is the main problem of billions of persons in the underdeveloped world, while excessive caloric intake leading to obesity is becoming more and more prevalent in Western societies of affluence. As a result, obesity, which leads to the metabolic syndrome and is thus closely associated with significant morbidity and mortality from diabetes, cardiovascular diseases, and cancers, to mention a few, is considered the epidemic of our century in Western societies.

Positive energy balance, as reflected by increasing BMI, is not a recent phenomenon. BMI has been increasing for many decades, but until the mid or late 1970s, it was rather associated with improved health and increased longevity. In the past few decades, however, the risk-to-benefit ratio has been shifting in such a way that the continued increase in body fatness is increasingly being recognized as underlying several chronic disease states. This phenomenon is slowing or even reversing gains made in terms of life expectancy in the past. More than 30% of Americans are currently overweight and another 30% are obese, defined as a body mass index (BMI) between 25.0 and 29.9 kg m-2 and higher than 30.0 kg m-2 respectively. Moreover, if the current trends continue, it is expected that by the year 2020 more than 50% of Americans will be obese, possibly making obesity the "norm" and leanness the "exception." In children, use of the term overweight is usually preferred, to avoid potential stigmatization, and thus the definition of obesity in children is based on exceeding the 95th percentile of BMI-for-age using the 2000 Centers for Disease Control charts.

Obesity is currently considered as being responsible for increasing morbidity as well as mortality, i.e., for the deaths of several hundreds of thousands of persons every year in Western societies. This fact makes obesity the second important potentially preventable cause of death after smoking. In addition to leading to illness, obesity can reduce significantly functional capacity and can increase disability. Realization of the above has prompted a heightened research interest in the factors influencing energy balance, and intensified research efforts on the links between obesity and its complications. It has also created an increasing demand for the study of new methods to diagnose, prevent, or treat obesity and associated comorbidities.

Negative energy balance, either due to lack of availability of appropriate nutrition leading to starvation in underdeveloped nations, or due to voluntary (dieting for weight loss) or involuntary caloric restriction (anorexia nervosa, exercise-induced or hypotha-lamic amenorrhea) in developed nations, is also of increasing prevalence. Immune dysfunction as well as certain well-defined neuroendocrine abnormalities leading to important adverse health consequences such as osteoporosis and infertility are the end result of energy deprivation. Research efforts to identify missing links between energy deficiency and these pathophysiological abnormalities have also been intensified over the past several years. In the area of epidemiology of obesity, the good news is that increasing rates of obesity appear to be reaching a plateau either because public health campaigns and interventions have started working and/or because almost all people with the genetic potential to develop obesity upon exposure to adverse environmental and dietary factors have already developed obesity. The bad news is that the prevalence of obesity continues to rise around the world and that this rising prevalence of obesity is associated with increasing rates of disability, morbidity, and mortality.

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